This invention relates to an exhaust device for a 2-cycle engine.
Heretofore, it has been known in the field of 2-cycle engine production to utilize the gas inertia effect in the exhaust pipe for scavenging the cylinders and drawing fresh gas mixtures thereinto by suction, to increase the output power of engines. More specifically, by causing the combustion gas of high pressure to rapidly expand when flowing through the exhaust pipe immediately after one of the cylinders has started discharge of gas, it is possible to increase pressure differential between the tuner section (exhaust gas expanding section) and the cylinder, to thereby positively draw the combustion gas out of the cylinder by suction.
It is also known to suddenly reduce the cross-sectional area of the passageway at the downstream end of the exhaust pipe (which is normally an inlet portion of a connecting pipe connected to the muffler) to produce positive reflection waves in such a manner that the period for the positive reflection waves to reach the exhaust port of the 2-cycle engine from the exhaust pipe will coincide with the fresh gas-air mixture blow-by period between the closure of the cylinder after scavenging and the closure thereof after exhausting, to thereby avoid the occurrence of the blow-by of the fresh gas-air mixture.
Proposals have hitherto been made to use an exhaust device for a 2-cycle engine shown in FIG. 6, for example, in which a 2-cycle engine is formed with exhaust ports b having an exhaust pipe d connected thereto which has a tuner section (expansion chamber) c of a predetermined length for the purpose of increasing the output power of the engine. The exhaust pipe d has a muffler e connected thereto through a connecting pipe f extending axially of the tuner section c. FIG. 7 shows another exhaust device which has also been proposed, in which a muffler g is mounted in such a manner that it encloses a part of the tuner section of the exhaust pipe d on the downstream side thereof and is located coaxially of the tuner section c.
An exhaust device shown in FIG. 8 has also been proposed, in which the connecting pipe f for the tuner section c of the exhaust pipe d and a muffler, not shown, includes an upstream end portion h which is inserted in the tuner section c, to thereby achieve the effect of reducing noise to a certain degree as compared with a 2-cycle engine having no connecting pipe and to achieve the effect of reducing power loss in the tuner section c. The noise reducing effect achieved by this construction would be accounted for by the fact that the noise produced by the exhaust could be greatly reduced by matching the resonance frequency produced by the length (volume) of an air chamber in the rear portion of the expansion chamber (tuner section c) determined by the length L1 of the upstream end portion h (inserted portion) of the connecting pipe f, with the fundamental and harmonic frequencies of the exhaust gas of a 2-cycle engine.
Some disadvantages are associated with these exhaust devices of the prior art. For example, the arrangement whereby the muffler e, g is located axially of the exhaust pipe d increases the overall length of the exhaust devices.
When the need arises to mount an exhaust device longitudinally as of a motorcycle A shown in FIG. 10, the muffler e of the exhaust device projects rearwardly of the motorcycle. In the case of a snowmobile B shown in FIG. 11 in which it is required to mount an exhaust device in an engine room i of limited space, it is necessary to bend the exhaust pipe d greatly to ensure that the necessary axial length is obtained, so that stable mounting of the exhaust device on the motorcycle is unobtainable.
Another proposal that has hitherto been made involves an exhaust device in which, as shown in FIG. 9, a muffler, not shown, is arranged through a connecting pipe n at one side of a tuner section m of an exhaust pipe k. However, this exhaust device would have the following disadvantages. Since the exhaust gas stream is discharged sideways after it has its pressure converted into static pressure following stagnation in the tuner section m, the output power of the engine would be greatly reduced. The rear portion of the tuner section m could not function sufficiently as a resonance chamber to reduce noise, so that the results achieved by the device in reducing noise would be poor.